The invention relates to a process for producing diamines and polyamines of the diphenylmethane series (MDA) by reacting aniline and formaldehyde in the presence of an acid catalyst, wherein the aniline used contains in total less than 0.5 wt. %, based on the weight of aniline used, of compounds which contain at least one carbonyl group or of compounds which are formed by reaction of these compounds containing at least one carbonyl group with aniline.
The compounds containing carbonyl groups and/or compounds formed therefrom by reaction with aniline may derive from the freshly used aniline, are contained in the recycled aniline or are formed as secondary products during the course of production of diamines and polyamines of the diphenylmethane series. They can optionally accumulate in the aniline recycle streams and reach contents above those in the fresh aniline.
Diamines and polyamines of the diphenylmethane series (MDA) are understood to be amines and mixtures of amines which correspond to the following structure:
where n stands for a natural number ≧2.
For compounds and mixtures of compounds with n=2, the term monomeric MDA (MMDA) is also conventionally used, while for compounds and mixtures of compounds with n>2 the term polymeric MDA (PMDA) is also conventionally used. For the sake of simplicity, mixtures of compounds in which compounds with n=2 and n>2 occur side by side are conventionally grouped together under the term MDA (diamines and polyamines of the diphenylmethane series).
The continuous, discontinuous or semi-continuous production of diamines and polyamines of the diphenylmethane series is described in countless publications and patents (see, for example H. J. Twitchett, Chem. Soc. Rev. 3(2), 209 (1974); W. M. Moore in: Kirk-Othmer Encycl. Chem. Technol., 3rd Ed., New York, 2, 338-348 (1978); EP-A-31 423; EP-B-1 167 343; EP-A-1 403 242; EP 934 922 B1).
In the methods used in industry, MDA is conventionally produced by reacting aniline and formaldehyde in the presence of acid catalysts, wherein at the end of the process the acid catalyst is conventionally neutralised by addition of a base, the reaction mixture is separated into an organic and an aqueous phase and the organic phase is transferred to the subsequent processing steps, such as, for example, removal of excess aniline by distillation (see U.S. Pat. No. 5,310,769; DE-A-198 04 918; and JP-A-2004026753).
Common to all processes described in the literature for producing MDA by reacting aniline and formaldehyde in the presence of an acid catalyst is the fact that during the reaction, chromophores are formed which discolor the MDA that is produced. If these discolorations are not sufficiently reduced or removed during neutralisation of the acid catalyst and removal of the aniline used in excess in the reaction, then during the subsequent phosgenation of the MDA to form the corresponding diisocyanates and polyisocyanates of the diphenylmethane series and their subsequent processing (e.g. separation of the solvent, separation of monomeric MDI), they often lead to darkly discolored products, which in turn give rise to yellowish discolored polyurethane foams or other discolored polyurethane (PU) materials. Although the inherent color of the diisocyanates and polyisocyanates of the diphenylmethane series does not negatively influence the mechanical properties of the polyurethanes produced therefrom, light-colored products are preferred because of their good variability in the processor's production process, with regard for example to showing through thin top coats and to color design possibilities. Accordingly, there has been no shortage of attempts to reduce the discoloration of MDA and the MDI produced therefrom.
EP 1 270 544 B1 describes a process for producing MDA with minimisation of the content of undesirable secondary products by reacting aniline with formaldehyde in the presence of acid catalysts, characterised in that in a semi-continuous process aniline and optionally acid catalyst are laid out, formaldehyde and optionally acid catalyst are fed through a mixing device into a circuit in which aniline, optionally acid catalyst and optionally pre-admixed formaldehyde are circulated, and after feeding in at least 50% of the total amount of formaldehyde to be introduced, the reaction mixture is heated to a temperature of greater than 75° C. The patent claims, in particular, the minimisation of the content of N-methyl MDA, the reduction of which in MDI should, according to the teaching of this patent, lead to a crude MDI of a lighter color in a subsequent phosgenation. The improvement of the color values by reducing the content of N-methyl MDA in the MDA that is produced, by means of a special formaldehyde feed, is the basis of the following process too.
DD-A-295 628 describes for a discontinuous process the addition of formaldehyde in two steps during the condensation stage, wherein the bulk of the formaldehyde is added in the first addition at low temperature and the second addition of the remaining formaldehyde takes place at the same or a higher temperature.
EP-A-451 442 and DD-A-238 042 disclose for a continuous process the addition of formaldehyde over several process stages.
In order to improve the color values, in addition to minimising the content of N-methyl MDA, U.S. Pat. No. 5,286,760 also teaches the minimisation of the secondary components acridan and acridine. However, U.S. Pat. No. 5,286,760 modifies not the formaldehyde feed but the molecular rearrangements following the primary reaction of the formaldehyde with aniline. U.S. Pat. No. 5,286,760 describes for a continuous MDA production a partial neutralisation of the reaction mixture between the condensation stage of two molecules of aniline and one molecule of formaldehyde and the subsequent molecular rearrangement of the intermediate aminobenzylamines (i.e. ABA) that are formed, to form MDA.
U.S. Pat. No. 5,310,769 likewise intervenes primarily in the molecular rearrangements. U.S. Pat. No. 5,310,769 describes a process for producing polyamines of the diphenylmethane series by condensation of aniline with formaldehyde, subsequent reaction in the presence of an acid catalyst, neutralisation of the acid catalyst on completion of the reaction and purification of the resulting diamine/polyamine mixture by removing the excess aromatic amine by distillation, characterised in that in a preferred variant    a) aniline is reacted with formaldehyde in a molar ratio of 1.5:1 to 10:1 at temperatures between 10 and 150° C.,    b) then an acid catalyst in the molar ratio of aniline to acid catalyst of 2:1 to 100:1 is added to the reaction mixture at temperatures of between 10 and 150° C., the water formed during the condensation reaction being separated off either before or after step b),    c) then the temperature of the mixture obtained in step b) is raised by at least 40° C. within 15 minutes and then heated further to the final temperature of between 105 and 200° C. and held at this temperature for 10 to 300 minutes.
U.S. Pat. No. 5,310,769 teaches that through the special temperature control during the condensation and molecular rearrangement steps, a diamine and polyamine mixture of the diphenylmethane series is obtained, the subsequent phosgenation of which provides access to especially light-colored polyurethane foams.
According to U.S. Pat. No. 4,792,624, diamine and polyamine mixtures of the diphenylmethane series, whose subsequent phosgenation leads to polyisocyanates with a greatly reduced coloration, are also obtained by application of a process characterised in that    a) fast-flowing streams of aqueous aniline hydrochloride and aqueous formaldehyde in a ratio of 1.6 to 8 moles of aniline per mole of formaldehyde are intensively mixed at the inlet of a tubular-flow reactor, as a result of which a mixture containing aminobenzylamines is immediately formed,    b) the mixture produced according to a) is subsequently passed through a cooled reaction section in which the content of aminobenzylamines in the mixture rises to at least 30 wt. %,    c) the reaction mixture is removed from the cooled reaction section at the rate at which reaction mixture flows in from step a),    d) the reaction mixture from the cooled reaction section is then passed through a rearrangement section with temperatures of 60° to 200° C., causing the polyamine of the diphenylmethane series to form,    e) the reaction mixture is removed from the rearrangement section at the rate at which reaction mixture is fed into the rearrangement section,    f) the reaction mixture from the rearrangement section is supplied continuously to a neutralisation section in which the acid components are neutralised, then aniline and water are separated from the reaction mixture so that an aniline-free polyamine of the diphenylmethane series is obtained,    g) the polyamine mixture is removed from step f) at the rate at which reaction mixture is fed into the neutralisation or distillation stage, and    h) the bulk of the polyamine mixture obtained is removed into a storage tank; however, a partial stream of the polyamine mixture of 1 to 40 wt. %, relative to the combined initial weights of the amounts of aniline, aniline hydrochloride and formaldehyde fed into step a), is returned to step b) and in the ongoing process a) to h) is passed once again through steps b) to h).
According to U.S. Pat. No. 4,792,624, in order to obtain a maximum improvement in the color values it is a substantial requirement for the excess aniline used in the process to be separated from the MDA before the MDA is added to the benzylamines. According to the teaching of U.S. Pat. No. 4,792,624, the improved coloration is moreover only achieved if the recycled polyamine is added at the point where aminobenzylamines are present, and not at the stage in which aniline and formaldehyde are reacted for the first time.
EP 1 813 598 A1 teaches that the use of aniline containing less than 3 wt. %, preferably 0.001 to 3 wt. %, particularly preferably 0.01 to 1 wt. % of diamines and polyamines of the diphenylmethane series, relative to the weight of the aniline used, in the production of MDA has an advantageous effect on the color of the MDI produced therefrom by phosgenation. The diamines and polyamines of the diphenylmethane series contained in the aniline find their way into the amine used for the production of MDA, since aniline is conventionally used in excess in MDA production and the excess is returned to the process after separation of the product by distillation, for example.
Other secondary components contained in the aniline are listed for example in RD 510004 (Research Disclosure Journal, published October 2006), such as, for example, cyclohexylamine, cyclohexanol, cyclohexanone, phenol, etc., without any mention of the contents in which these secondary components are included in the aniline and the influence, if any, these secondary components have on the quality of the MDA and on the MDI obtained therefrom.
Common to many of the processes cited above and described in the literature for the production of diamines and polyamines of the diphenylmethane series by reacting aniline with formalin in the presence of acid catalysts is the fact that by modification of individual process parameters, e.g. the component dosing, the concentration of acid catalyst, the temperature control or the product composition during the molecular rearrangements, they bring about improvements in the coloration of the diamine/polyamine mixtures of the diphenylmethane series that are produced or of the isocyanates and polyurethanes produced therefrom. Nevertheless, there is still a need for new processes with still further color improvements.
Surprisingly this objective could be achieved in a process for the production of diamines and polyamines of the diphenylmethane series by reacting aniline and formaldehyde in the presence of acid catalysts, in which the aniline used contains in total less than 0.5 wt. %, preferably 0.0001 to 0.4 wt. %, more preferably 0.0001 to 0.3 wt. % and most preferably 0.0001 to 0.25 wt. %, based on the weight of aniline used, of certain types of compounds as described herein.